Oh, now we understand. When the e-mail from Boston Casting arrived in our inbox — the one requesting “young men with awesome mohawks to work as extras” — we confess we were confused. Is someone shooting a remake of “Taxi Driver”? Or “Repo Man”? No, Boston Casting needed men with mohawks for a scene in director David O. Russell’s new movie. We finally figured that out when we saw pictures of Bradley Cooper squaring off against a spiky-haired dude in a faux fight as the cameras were rolling. The movie, whose A-list ensemble cast includes Cooper, Christian Bale, and Jennifer Lawrence, finally has a title. Columbia Pictures announced Tuesday that the film loosely based on the FBI’s 1970s sting operation known as Abscam, is called “American Hustle.”